In recent years, there has been an increased concern in the United States and elsewhere about air pollution from industrial emissions of noxious oxides of nitrogen, sulfur and carbon. In response to such concerns, government agencies have placed limits on allowable emissions of one or more of the pollutants, and the trend is clearly in the direction of increasingly stringent restrictions.
NOx, or oxides of nitrogen, in flue gas streams exiting from fluid catalytic cracking (FCC) regenerators is a pervasive problem. Fluid catalytic cracking units (FCCUs) process heavy hydrocarbon feeds containing nitrogen compounds a portion of which is contained in the coke on the catalyst as it enters the regenerator. Some of this coke nitrogen is eventually converted into NOx emissions, either in the FCC regenerator or in a downstream CO boiler. Thus all FCCUs processing nitrogen-containing feeds can have a NOx emissions problem due to catalyst regeneration.
In an FCC process, catalyst particles (inventory) are repeatedly circulated between a catalytic cracking zone and a catalyst regeneration zone. During regeneration, coke which deposits from the cracking reaction onto the catalyst particles is removed at elevated temperatures by oxidation with oxygen containing gases such as air. The removal of coke deposits restores the activity of the catalyst particles to the point where they can be reused in the cracking reaction. The coke removal step is performed over a wide range of oxygen availability conditions. At the minimum, there is typically at least enough oxygen to convert all the coke made to CO and H2O. At the maximum, the amount of oxygen available is equal to or greater than the amount necessary to oxidize all the coke to CO2 and H2O.
In an FCC unit operating with sufficient air to convert essentially all of the coke on the catalyst to CO2 and H2O, the gas effluent exiting the regenerator will contain “excess oxygen” (typically 0.5 to 4% of total off gas). This combustion mode of operation is usually called “complete” or “full burn”. When the fluid catalytic cracking unit (FCCU) regenerator is operating in full burn mode, the conditions in the regenerator are for the most part oxidizing. That is, there is at least enough oxygen to convert (burn) all reducing gas phase species (e.g., CO, ammonia, HCN, H2S and COS) regardless of whether this actually happens during the residence time of these species in the regenerator. Under these conditions, essentially all of the nitrogen deposited with coke on the catalyst during the cracking process in the FCCU riser is eventually converted to molecular nitrogen or NOx and exits the regenerator as such with the off gas.
Simultaneously with NOx emissions problems, afterburn and emissions of CO may also be a concern for full burn units. Gases exiting the catalyst bed of an FCCU operating in a full burn combustion mode will consist mainly of CO2, H2O, O2 (typically called excess oxygen), and minor amounts of CO, NO, SO2, and potentially some reduced nitrogen species. However, depending on the design and mechanical condition of the regenerator, conditions can develop in which sufficient amounts of CO and O2 escape the catalyst allowing the CO to react with the available O2. The reaction can occur in the regenerator at any point above the dense catalyst bed, including the area above the dense bed (dilute phase), the cyclones where entrained catalyst is separated from the flue gas, the plenum above the cyclones, or even the flue gas pipe.
This phenomenon, generally referred to as “afterburn”, is common in full burn regenerators because by the very nature of this mode of operation there is excess O2 in the regenerator available to “light up” CO escaping the dense bed. Because afterburn occurs after the dense bed of the cracking catalyst which acts as a heat sink absorbing the heat released from the exothermic reaction of CO with O2, it can heat up the gases to the point that overheating can occur. The result can be temperatures which approach the metallurgical limit of the materials used to construct the regenerator. High afterburn can limit the useful life of the regenerator equipment, and runaway afterburn can cause catastrophic equipment failure.
Further, unlike units operating in partial burn, or even in incomplete combustion mode, which typically have a CO boiler, full burn units do not generally have a fired boiler on the regenerator flue gas train. As a result, any CO escaping the regenerator will be emitted to the atmosphere. In many FCCUs operating in full burn combustion mode, CO combustion promoters are used to minimize the emission of CO from the regenerator by promoting the combustion of CO to CO2. While many CO combustion promoter formulations have been used, conventional CO combustion promoters typically comprise an additive comprised of 300 to 1000 ppm platinum or alumina, or much smaller amounts of platinum, e.g. amounts which typically achieve from about 0.1 to about 10 ppm in the total cracking catalyst inventory, are incorporated directly into the cracking catalyst itself. Unfortunately, however, such CO combustion promoter additives or formulations typically cause a dramatic increase (e.g. 300%) in NOx emissions from the regenerator.
Consequently, recent approaches for controlling industrial emissions from an FCCU have attempted to reduce the level of NOx emissions while simultaneously promoting CO combustion during an FCC catalyst regeneration step. For example, U.S. Pat. Nos. 6,165,933 and 6,358,881 disclose the use of a NOx reduction composition, which promotes CO combustion during an FCC catalyst regeneration process step while simultaneously reducing the level of NOx emitted during the regeneration step. NOx compositions disclosed by these patents may be used as an additive, which is circulated along with the FCC catalyst inventory or incorporated as an integral part of the FCC catalyst.
In U.S. Pat. No. 4,290,878, NOx is controlled in the presence of a platinum-promoted CO oxidative promoter in a full burn combustion regenerator. This patent requires the addition of iridium or rhodium on the combustion promoter in amounts lesser than the amount of platinum presence in the CO oxidative promoter.
U.S. Pat. No. 4,973,399 discloses the use of copper-loaded zeolite additives for reducing emissions of NOx from the regenerator of an FCCU unit operating in full CO-burning mode.
However, there still remains a need in the refining industry for improved FCC processes which minimize the content of NOx emitted from an FCC regenerator operating in a full burn or complete burn mode and simultaneously promote the combustion of CO.